A data processing apparatus which executes data processing instructions in order to carry out, or delegate, data processing operations is known to be arranged to be responsive to a barrier instruction. A barrier instruction causes the data processing apparatus to ensure that a particular access ordering constraint is enforced, that is to say the barrier instruction ensures that instructions which precede the barrier instruction in the sequence of data processing instructions which the data processing apparatus executes complete, i.e. can be assured take effect within the memory system accessed, before the data processing apparatus can execute instructions in the sequence of data processing instructions which follow the barrier instruction. This can for example be useful to ensure that the content of the memory system is up-to-date and coherent with respect to the current processing status of the data processing apparatus when it encountered the barrier instruction.
The present disclosure relates to improvements in how the data processing apparatus responds to encountering such a barrier instruction.